1. Field of the Invention
The present invention relates to a bicycle drive unit, especially to a bicycle drive unit that is mounted onto a bicycle frame, and supports a rear wheel.
2. Background Information
A bicycle is usually equipped with a frame and a drive component that includes right and left cranks mounted onto the frame in a freely rotatable manner. Front and rear wheels are typically mounted onto front and rear portions of the frame.
Some bicycles include a modular drive unit that is connected to only the drive component and a mid-portion of the frame, with the rear wheel mounted to the modular drive unit instead of the rear portion of the frame. An example of such a modular drive unit is well known in, for instance, Official Gazette of Germany No. 19840576. By utilizing a bicycle drive unit, the assembling efficiency of such a bicycle is improved, and assembling costs are reduced. Furthermore, because the chain is concealed within the modular drive unit, it is not exposed to the elements. Further, since the chain is not exposed, the chain cannot stain a bicycle rider's clothes, and is also maintenance-free.
A typical bicycle drive unit, such as that disclosed in Official Gazette of Germany No. 19840576, includes a supporting frame or case frame that serves as a rear portion of the frame. Specifically, the rear wheel is completely supported on the case frame. A forward portion of the case frame is pivotally fixed to a mid-portion of the bicycle frame and is also fitted with a crank shaft. Right and left ends of the crank shaft are fitted with right and left cranks that the bicycle rider engages to generate rotary power for the bicycle. A rear hub is fitted in a rearward portion of the case frame with the rear wheel mounted to the rear hub. The case frame is at least partially hollow. A front sprocket is fitted onto the right crank and is disposed within the case frame adjacent to the right crank. A rear sprocket is mounted onto the rear-hub and is also disposed within the case frame. A chain within the case frame wraps around the front and rear sprockets for transmitting rotary power therebetween.
The case frame undulates with respect to the bicycle frame so as to move like a see-saw pivoting in response to bumps on the road or the like. The case frame includes a cylindrical crank supporting portion that extends laterally outwardly in order to support right and left bearings that in turn support the crank shaft. The front sprocket is located on an outward side of the right side bearing. Specifically, the front sprocket is not located between the right and left bearings, but is located on an outboard side of the right side bearing on the crank shaft adjacent to the right crank.
In the conventional bicycle drive unit, the front sprocket is mounted onto the right crank at a position outward from the right side bearing. As a result, it is necessary to design the crank shaft and cranks with considerable strength due to the torsion angles of the crank shaft and cranks. Specifically, the distance between end of the left crank and the front sprocket is such that considerable stress placed upon the crank shaft and crank. Therefore, the size of the crank shaft and crank must be increased in order to withstand typical pedaling forces, thereby increasing the overall mass of the bicycle. Furthermore, the width of the case frame is relatively narrow with inclusion of the crank supporting portions that extend laterally outward. This reduces the torsional rigidity of the case frame. Therefore, any efforts to maintain torsional rigidity of the case frame would typically require a corresponding increase in the weight of the case frame, which is counter to current efforts to decrease the weight of bicycles.
Moreover, because the crank extends from the front sprocket, a large opening must be formed in the side of the case frame. Therefore, demands on seals of the case frame are increased, typically contributing to the leakage of lubricant out of the case frame.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved modular drive unit that includes a design that increases torsional stability and sealability of the drive unit. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.